The invention pertains to optical borosilicate glasses containing high zirconium concentrations and having intermediate to high refractive indices and relatively low Abbe numbers, as well as to a process for the production of such glass compositions.
The demand for optical glasses with special physical properties on the part of optical engineers has risen strongly in recent times. For the redesign of optical systems, particularly microscope and photographic lenses, optical grade glasses characterized by high refractive indices n.sub.e but at the same time possessing the lowest possible Abbe numbers .nu..sub.e are increasingly required. By using such special glasses, optical engineers are able to eliminate optical abberations in optical systems and to improve the corrective quality of an objective to a much higher degree. Anomalous partial dispersion values play a decisive role in this respect. U.S. Pat. No. 3,451,829 recites additional details concerning the significance, the application and the nomenclature of optical glasses having anomalous partial dispersions.
In addition to these purely physical properties which are demanded of advanced optical glasses, those skilled in the glass art must also consider chemical-industrial requirements, i.e., the glasses must be capable of being produced on an industrial scale and they must not have any distinct crystallization tendencies. Economical production requires melting temperatures which are not excessive. Finally, technical processing factors play an important role in the selection and use of glasses of this type.
Even though glasses with compositions containing essentially silicon dioxide (SiO.sub.2), boron trioxide (B.sub.2 O.sub.3), zirconium dioxide (ZrO.sub.2) and alkaline earth metal oxides are known, such glasses have proven to be unsuited for industrial application, because, as a result of their distinct crystallization tendencies, they may be melted only in small and therefore uneconomical units. Were one to produce comparable glasses containing alkali metal oxides in place of alkaline earth metal oxides, the negative deviations from the so-called "standard straight line," i.e., the (-).DELTA..nu..sub.e values, would be significantly lower.
Furthermore, U.S. Pat. No. 2,150,694 describes a process for the manufacture of optical glasses having high refractive indices and low scatter, which contain, inter alia, one or more oxides of the elements titanium, yttrium, zirconium, niobium, lanthanum, tantalum and thorium. Further additives may consist of thorium and hafnium. However, such known glasses have different values in their n.sub.e /.nu..sub.e pairs of parameters. Also, no data are given concerning the required anomaly of the partial dispersion.